The present invention relates to an improved bipolar electrolyzer adapted for the use in the production of oxyhalogen compounds such as sodium chlorate by electrolysis of an alkali metal halide such as sodium chloride. More specifically the instant invention concerns a bipolar electrolyzer which is characterized by its low voltage drop, ease of maintenance and relative low cost.
Multielectrode electrolytic cells including bipolar electrodes have been used for the production of oxyhalogen compounds due to the fact that this type of cell is compact and does not require electric current leads and exposed metallic members connecting the bus bars to the intermediate electrodes. Electrical connections need be made only to the terminal electrodes.
In the production of sodium chlorate, sodium chloride in an aqueous solution is disassociated by electrolytic action. In the absence of an anode to cathode separator, or diaphragm, these disassociated ions immediately recombine by chemical action to form sodium hypochlorite and hydrogen gas shown by the following equations: ##STR1##
By a much slower process the hypochlorite thus formed decomposes to chlorate as shown by the following equation: EQU 3NaOCl.fwdarw.NaClO.sub.3 +2NaCl1
In order to maintain good current efficiency and optimum reaction conditions the electrolyzer is generally positioned within a reaction tank. To assure optimum operating conditions the electrolyte should circulate rapidly and turbulently through the cell to insure efficient generation of hypochlorite and then circulate between the reaction tank and electrolytic cell, at a rate which provides sufficient residence time for conversion of the hypochlorite to chlorate in the reaction tank. The generation of hydrogen gas at each cathode surface in the electrolytic cell provides the driving force to circulate the electrolyte through the individual cell compartments in a parallel flow pattern.
Considerable heat is generated during the electrolysis in the cell units and to insure proficient performance of the cells and for stability of the materials of construction it is necessary to provide for removal of the generated heat. Cooling coils are generally immersed in the reaction or holding tank to maintain suitable operating temperatures.
When suitable chemical and pH conditions are maintained in the operation of electrolytic cells, current efficiency is dependent primarily on the rate of flow of the electrolyte solution through the cell units, the current leakage loss and holding tank residence time.
To minimize current leakage each bipolar electrode is provided with an electrical insulating spacer on each end. These non-conducting spacers have a horizontal groove or channel so that they may be slipped over the end of the electrode and thus held in place. When these electrodes are stacked in an overlaying vertical stack, the insulating spacers which are located at the midpoint of alternate electrodes provide for both spacing of anode and cathode and for the isolation of each cell compartment. Each of these cell compartments which are arrayed in series electrically may house as many horizontally arrayed bipolar electrodes in the vertical stack as is necessary to achieve the capacity desired. Openings are provided in the top and bottom of the cell box in sufficient number for each compartment to allow for inlet and outlet flow of electrolyte.
U.S. Pat. No. 3,819,503 describes a bipolar cell which has improved operating characteristics. However, this cell design utilizes bipolar electrode assemblies which are formed by bolting together stacks of monopolar electrodes to form bipolar electrodes. In this type of arrangement, it is difficult to maintain constant spacing between electrodes. Cost is greater and voltage drop, and thus power consumption; is higher.
Accordingly, it is an object of the present invention to provide an improved bipolar electrolytic cell configuration wherein a bipolar electrode assembly is employed which is not formed of monopolar electrodes bolted together and which has associated therewith a means for maintaining the spacing between individual electrodes in the assembly.
Another object of the invention is to provide an improved means of mounting the monopolar terminal electrodes in the terminal compartments of the electrolytic cell.
These and other objects of the invention will be apparant to those skilled in the art from a reading of the following specification and claims.